Ceramic honeycombs and metal honeycombs composed of a stainless steel foil have been widely used as a catalyst carrier for exhaust gas purifying facilities included in automobiles, agricultural machinery, building machinery, industrial machinery, and the like. Among these honeycombs, recently, metal honeycombs have been increasingly used since they allow a higher aperture ratio to be achieved and have higher resistance to thermal shock and higher vibration resistance than ceramic honeycombs.
A metal honeycomb has a honeycomb structure formed by, for example, stacking a flat stainless steel foil (flat foil) and a stainless steel foil that has been worked into a corrugated shape (corrugated foil) alternately. A catalytic material is applied onto the surface of the stainless steel foil, and the resulting metal honeycomb is used in an exhaust gas purifying facility. When a catalytic material is applied onto the surface of the stainless steel foil, the stainless steel foil is commonly coated with γ-Al2O3 to form a wash coat layer and a catalytic material such as Pt or Rh is applied to the wash coat layer.
FIG. 1 illustrates an example of a metal honeycomb. The metal honeycomb illustrated in FIG. 1 is a metal honeycomb 4 prepared by stacking a flat foil 1 and a corrugated foil 2 composed of a stainless steel foil, winding the resulting product into a roll shape, and fixing the periphery of the wound product in place with an external cylinder 3 composed of a stainless steel.
Because the metal honeycomb is exposed to a high-temperature exhaust gas, a material of the metal honeycomb, that is, a stainless steel foil, is required to have high oxidation resistance. The material of the metal honeycomb, that is, the stainless steel foil, is also required to have high adhesion (adhesion to a catalyst coat) to a catalyst coat (wash coat layer on which a catalytic material is deposited).
For the above-described reasons, hitherto, high-Al-content ferritic stainless steel foils such as a 20 mass % Cr-5 mass % Al ferritic stainless steel foil and a 18 mass % Cr-3 mass % Al ferritic stainless steel foil have been primarily used as a stainless steel foil to form a catalyst carrier for exhaust gas purifying facilities such as a metal honeycomb.
When Al is added to a stainless steel such that the Al content in the stainless steel is 3 mass % or more, the surface of the stainless steel can be protected by an Al oxide layer mainly composed of Al2O3, which markedly enhances oxidation resistance. Moreover, corrosion resistance at high temperatures can also be markedly enhanced. The Al oxide layer has a high affinity for a γ-Al2O3 coat (wash coat) commonly used to deposit a catalyst on the foil and, therefore, has high adhesion to a catalyst coat (adhesion between the oxide layer and the wash coat). Thus, a high-Al-content ferritic stainless steel foil having an Al content of 3 mass % or more has markedly high adhesion to a catalyst coat.
High-Al-content ferritic stainless steel foils have been widely used as a material of a catalyst carrier since they have high oxidation resistance and high adhesion to a catalyst coat. In particular, exhaust gas purifying facilities of gasoline-powered automobiles, in which the temperature of the exhaust gas reaches 1000° C. or more, include a catalyst carrier composed of a 20 mass % Cr-5 mass % Al ferritic stainless steel foil or a catalyst carrier composed of a 18 mass % Cr-3 mass % Al ferritic stainless steel foil, which have markedly high oxidation resistance.
On the other hand, the temperature of exhaust gas of diesel-powered automobiles does not increase as high as the temperature of exhaust gas of gasoline-powered automobiles, and the temperature reached is generally about 800° C. The highest temperature reached by exhaust gas of agricultural machinery, building machinery, industrial machinery, a factory or the like is even lower than the highest temperature reached by exhaust gas of diesel-powered automobiles. Therefore, a material of a catalyst carrier for exhaust gas purifying facilities included in diesel-powered automobiles, industrial machinery and the like, in which the temperature of exhaust gas is relatively low, is not required to have markedly high oxidation resistance comparable to those of a 20 mass % Cr-5 mass % Al ferritic stainless steel foil and a 18 mass % Cr-3 mass % Al ferritic stainless steel foil.
Furthermore, the production efficiency of a high-Al-content ferritic stainless steel foil having an Al content of 3 mass % or more is low, which increases the production cost, while the high-Al-content ferritic stainless steel has high oxidation resistance and high adhesion to a catalyst coat. Because adding a large amount of Al to a ferritic stainless steel significantly reduces the toughness of the ferritic stainless steel, cracking may occur while a cast slab is cooled, and rupturing of a steel sheet may often occur during a treatment of a hot-rolled sheet or during cold rolling performed in the production of the high-Al-content ferritic stainless steel foil. This results in difficulty in producing the foil and a reduction in yield. Moreover, hard oxide scale may be formed on a high-Al-content steel, which deteriorates the product quality in a descaling step in which pickling, polishing and the like are performed and increases the number of man-hours required.
To address the above-described problems, techniques have been proposed in which the production efficiency of a ferritic stainless steel foil used as a material of a catalyst carrier such as a metal honeycomb is improved by reducing the Al content in the foil to a minimum.
For example, Japanese Unexamined Patent Application Publication No. 7-213918 proposes a technique in which a metal honeycomb is formed by stacking a flat sheet and a corrugated sheet composed of a ferritic stainless steel foil alternately by diffusion bonding or liquid-phase bonding, the ferritic stainless steel foil having an Al content limited to an impurity level to 0.8% in terms of weight proportion and a Nb content of 0.1% to 0.6%. According to the technique proposed in Japanese Unexamined Patent Application Publication No. 7-213918, it is possible to improve the production efficiency of the ferritic stainless steel foil while achieving high oxidation resistance of the foil. Furthermore, it is possible to reduce the risk of formation of an alumina layer, which inhibits bonding when a heat treatment is performed at a high temperature during diffusion bonding or liquid-phase bonding. This enables a metal honeycomb to be produced at a low cost.
Japanese Unexamined Patent Application Publication No. 7-275715 proposes a technique in which a metal honeycomb is formed by stacking a flat sheet and a corrugated sheet composed of a ferritic stainless steel foil alternately by diffusion bonding or liquid-phase bonding, the ferritic stainless steel foil having an Al content limited to an impurity level to 0.8% in terms of weight proportion and a Mo content of 0.3% to 3%. According to the technique proposed in Japanese Unexamined Patent Application Publication No. 7-275715, it is possible to improve the production efficiency of the ferritic stainless steel foil while achieving high oxidation resistance of the foil and high resistance to sulfuric acid corrosion of the foil. In addition, it is possible to reduce the risk of formation of an alumina layer, which inhibits bonding when a heat treatment is performed at a high temperature during diffusion bonding or liquid-phase bonding. This enables a metal honeycomb to be produced at a low cost.
Japanese Unexamined Patent Application Publication No. 2004-307918 proposes a technique not related to a stainless steel foil but to an Al-containing ferritic stainless steel sheet having a thickness of about 0.6 to 1.5 mm used as a material of a catalyst-carrying member in which Al is added to a 18 mass % Cr steel such that the Al content in the steel is 1.0% to less than 3.0% by mass % and an oxide layer having an Al content of 15% or more and a thickness of 0.03 to 0.5 μm is formed on the surface of the steel sheet. According to the technique proposed in Japanese Unexamined Patent Application Publication No. 2004-307918, it is possible to produce an Al-containing heat-resistant ferritic stainless steel sheet having high workability and high oxidation resistance.
However, in the techniques proposed in Japanese Unexamined Patent Application Publication No. 7-213918 and Japanese Unexamined Patent Application Publication No. 7-275715, since the Al content in the ferritic stainless steel foil is reduced to 0.8% or less in terms of weight proportion, an Al oxide layer cannot be formed on the surface of the foil at high temperatures, but a Cr oxide layer is formed instead. If a Cr oxide layer is formed instead of an Al oxide layer, the oxidation resistance of the ferritic stainless steel foil may be degraded. In addition, if a Cr oxide layer is formed instead of an Al oxide layer, shape stability of the ferritic stainless steel foil at high temperatures and adhesion of the foil to an oxide layer (adhesion between a base iron and the oxide layer) may be degraded, which results in degradation of the adhesion of the foil to a catalyst coat (adhesion between the oxide layer and the wash coat).
If the oxide layer formed on the surface of the foil is composed of a Cr oxide layer only, the difference in thermal expansion coefficient between the oxide layer and a base iron becomes large compared to when the oxide layer is composed of an Al oxide layer. As a result, creep deformation may occur at a high temperature, which results in deformation of the foil and peeling of the oxide layer. In addition, when a catalytic material is applied onto the surface of such a ferritic stainless steel foil, the catalyst coat deposited on the surface of the ferritic stainless steel foil may become detached due to the deformation of the foil and peeling of the oxide layer that may occur at a high temperature. Thus, it is impossible to produce a metal honeycomb having the properties required for a catalyst carrier by the techniques proposed in Japanese Unexamined Patent Application Publication No. 7-213918 and Japanese Unexamined Patent Application Publication No. 7-275715.
The technique proposed in Japanese Unexamined Patent Application Publication No. 2004-307918 is directed to a cold-rolled steel sheet having a thickness of 1 mm. Thus, a foil material suitable as a material of a catalyst carrier is not always produced by applying this technique to a foil material. Since a foil material is considerably thin, the high-temperature strength of a base iron of a foil material is lower than that of a plate material, and a foil material is likely to be deformed at a high temperature. Therefore, when the technique proposed in Japanese Unexamined Patent Application Publication No. 2004-307918 is applied to a foil material, deformation may occur due to the difference in thermal expansion coefficient between the oxide layer and the base iron when Al is depleted and a Cr oxide layer begins to be formed while the foil material is oxidized at a high temperature because the proof stress of the base iron of the foil material is not sufficiently high.
Furthermore, when a stainless steel having an Al content of less than 3% is oxidized at a high temperature, an Al oxide layer is not formed on the surface of the stainless steel consistently, which significantly deteriorates adhesion to a catalyst coat. In general, a Cr oxide layer mainly composed of Cr2O3 is formed on the surface of a stainless steel having an Al content of less than 3% at a high temperature. However, Cr2O3 has poor adhesion to γ-Al2O3, which constitutes a wash coat (adhesion to a catalyst coat). Moreover, as described above, deformation may occur due to the difference in thermal expansion coefficient between the Cr oxide layer and the base iron, and peeling of the wash coat and the deposited catalyst is likely to occur.
As described above, degradation of oxidation resistance, shape stability at high temperatures, adhesion to an oxide layer, and adhesion to a catalyst coat, which may be caused due to formation of a Cr oxide layer, have been serious problems for a ferritic stainless steel foil in which the Al content is reduced to improve the production efficiency and workability of the foil.
It could therefore be helpful to provide a ferritic stainless steel foil suitable as a material of a catalyst carrier for exhaust gas purifying facilities (e.g., metal honeycomb) which are used at relatively low temperatures, that is, specifically, to improve the oxidation resistance of a low-Al ferritic stainless steel foil, the shape stability of the foil at high temperatures, the adhesion of the foil to an oxide layer, and the adhesion of the foil to a catalyst coat and to provide a ferritic stainless steel foil having good production efficiency.